Chapter 8. Database Records

Chapter 8. Database Records
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Table of Contents

Reading and Writing Database Records

Writing Records to the Database
Getting Records from the Database
Deleting Records
Data Persistence

Numerical and String Objects
Serializable Complex Objects
Custom Tuple Bindings

DB records contain two parts — a key and some data. Both the key and its corresponding data are encapsulated in DatabaseEntry class objects. Therefore, to access a DB record, you need two such objects, one for the key and one for the data.

DatabaseEntry can hold any kind of data from simple Java primitive types to complex Java objects so long as that data can be represented as a Java byte array. Note that due to performance considerations, you should not use Java serialization to convert a Java object to a byte array. Instead, use the Bind APIs to perform this conversion (see Using the BIND APIs for more information).

This chapter describes how you can convert both Java primitives and Java class objects into and out of byte arrays. It also introduces storing and retrieving key/value pairs from a database. In addition, this chapter describes how you can use comparators to influence how DB sorts its database records.

Using Database Records

Each database record is comprised of two DatabaseEntry objects — one for the key and another for the data. The key and data information are passed to- and returned from DB using DatabaseEntry objects as byte arrays. Using DatabaseEntrys allows DB to change the underlying byte array as well as return multiple values (that is, key and data). Therefore, using DatabaseEntry instances is mostly an exercise in efficiently moving your keys and your data in and out of byte arrays.

For example, to store a database record where both the key and the data are Java String objects, you instantiate a pair of DatabaseEntry objects:

package db.GettingStarted;

import com.sleepycat.db.DatabaseEntry;

...

String aKey = "key";
String aData = "data";

try {
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
} catch (Exception e) {
    // Exception handling goes here
}

    // Storing the record is described later in this chapter

Note

Notice that we specify UTF-8 when we retrieve the byte array from our String object. Without parameters, String.getBytes() uses the Java system's default encoding. You should never use a system's default encoding when storing data in a database because the encoding can change.

When the record is retrieved from the database, the method that you use to perform this operation populates two DatabaseEntry instances for you, one for the key and another for the data. Assuming Java String objects, you retrieve your data from the DatabaseEntry as follows:

package db.GettingStarted;

import com.sleepycat.db.DatabaseEntry;

...

// theKey and theData are DatabaseEntry objects. Database
// retrieval is described later in this chapter. For now, 
// we assume some database get method has populated these
// objects for us.

// Use DatabaseEntry.getData() to retrieve the encapsulated Java
// byte array.

byte[] myKey = theKey.getData();
byte[] myData = theData.getData();

String key = new String(myKey, "UTF-8");
String data = new String(myData, "UTF-8");

There are a large number of mechanisms that you can use to move data in and out of byte arrays. To help you with this activity, DB provides the bind APIs. These APIs allow you to efficiently store both primitive data types and complex objects in byte arrays.

The next section describes basic database put and get operations. A basic understanding of database access is useful when describing database storage of more complex data such as is supported by the bind APIs. Basic bind API usage is then described in Using the BIND APIs.

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